Method and device for producing continuous belts from plastic films

ABSTRACT

A method and apparatus for producing continuous belts is disclosed. The belts are formed from plastic films and are used as transfer belts in electrographic printers and copiers. The ends of the film are welded together by abutting their front faces and the ends are held together under pressure while being heated by radiation to a temperature to cause welding of the ends.

BACKGROUND OF THE INVENTION

[0001] 1.Field of the Invention

[0002] The present invention is directed to a method and to a device formanufacturing an endless band of plastic for an intermediate carrierband in an electrographic printer or copier.

[0003] 2. Description of the Related Art

[0004] Intermediate carrier bands are utilized in electrographicprinters or copiers in order to generate latent electrostatic imagesand/or to offer a carrier for toner images to be transfer-printed. Forexample, an endless band with a photoconductive layer, for example anOPC band (Organic PhotoConducting material) is employed as intermediatecarrier band, this forming a corresponding electrostatic charge image,what is referred to as a latent charge image, by being exposed accordingto a predefined image pattern. This latent charge image is then inkedwith toner material in a developer station; later, this toner image istransferred onto paper or some other recording medium and is fixedthereon.

[0005] An endless intermediate carrier band can also serve as transferband for collecting toner images and conveying these to a transferprinting location. Given, for example, a multi-color printing, a firsttoner image of a first color is transferred onto the intermediatecarrier band. Subsequently, a second toner image with a second color istransferred onto this first toner image, etc. The multi-colored tonerimages on the intermediate carrier band superimposed on one another inthis way are then conveyed to a transfer printing station andtransferred onto the recording medium thereat and fixed.

[0006] Ends of a plastic film must be connected to one another formanufacturing an endless intermediate carrier band. The weld thatthereby arises can be the cause of numerous disruptions. For example, athickening along the weld leads to increased wear due to circulation ofthe intermediate carrier band. Moreover, the material properties canhave been changed in the region of the weld, so that this region cangenerally not be used as a photoconductive region or as a region for theacceptance of a toner image.

[0007] German Patent Document DE 19 832 168 A1 discloses a method and anapparatus for welding thermoplastic synthetics using laser light. Theends of a thermoplastic plastic film are arranged abutting and can beheld with the assistance of a retainer elements and a silica glassplate. Laser light is coupled in via the silica glass plate, as a resultwhereof the ends of the film are welded to one another. Special measuresfor producing a uniform weld are not disclosed.

[0008] German Patent Document DE 19 516 726 A1 discloses a method forshaping and closing a folding box, whereby plastic layers for packing awelded to one another upon employment of radiation. The welding processis promoted by applying pressure.

[0009] German Patent Document DE 3 713 527 A1 discloses the welding ofplastic parts whose ends are place flush against one another. Theplastic parts are provided with profiles at their ends, so that theseprofiles can engage in one another. The ends with the profiles are thenwelded to one another with the assistance of a laser welding device.

[0010] European Patent Document EP-A-0 705 682 discloses a method forthe thermal joining of substrates of polymers, whereby at least onesubstrate is coated with a medium that absorbs microwaves. Thesubstrates are then welded to one another in a microwave field.

[0011] Internet information of the EWi WELDNET company with the title“Hot Plate Welding”, obtainable underhttp:H/www.ewi.org/matjoin/plastics/ttir.html;http://www.ewi.org/matjoin/plastics/hotplate.html;http://www.ewi.org/matjoin/plastics/infrared.html, teaches that plasticparts be firmly joined to one another by means of a butt joint welding.A heating element is thereby placed between the contact surface of theparts to be welded, the surfaces lying opposite one another at their endfaces, and the contact surface are heated up to the melting phase.Subsequently, the heating element is removed and the contact surfacesresiding opposite one another are pressed against one another. A durableconnection between the contact surfaces is provided after cooling. Sucha method, however, has the disadvantage that a raised weld projectingout of the surfaces of the parts welded to one another arises at thecontact surfaces.

[0012] An overlap welding method is disclosed under the titles“Through-transmission Infrared Welding TTIR” or, respectively, “InfraredHeating” in the aforementioned Internet information. The ends of plasticparts to be welded are thereby placed on top of one another and heatedup to the melting phase by infrared radiation or laser emission from aradiation source that is not in contact with the parts to be welded. Oneapplication of this method is the welding of a part that is transparentfor the radiation to a part that is impermeable to the radiation,whereby the welding arises in the region of the surfaces of the parts tobe welded that lie on top of one another. A further application of thismethod is the welding of plastic films lying on top of one another atends with the assistance of a thin intermediate layer. The method is notsuitable for the manufacture of endless bands.

SUMMARY OF THE INVENTION

[0013] The present invention provides a method and a device formanufacturing an endless band of thermoplastic synthetic whose surfacesallows high usage given low wear.

[0014] This is achieved for a method for manufacturing an endless bandof plastic for an intermediate carrier band in an electrographic printeror copier, whereby the ends of a thermoplastic plastic film thatcomprises at least the width of a standard printing format, have theirend faces placed abutting one another, the ends of the plastic film areheated by radiation to a temperature required for the welding, andwhereby a respective pressing surface is arranged at both sides of theends, the length of the pressing surface at least corresponding to thewidth if the plastic film and this pressing the surfaces of the endsagainst one another such that, when the plastic material of the endfaces of the ends residing opposite one another melts, the spacing ofthe pressing surfaces defined by the thickness of the cold plastic filmis preserved.

[0015] According to the invention, the pressing surfaces have a spacingfrom one another that is defined by the thickness of the cold plasticfilm. When the adjoining ends of the plastic film are heated, the moltenmass cannot become thicker than prescribed by the spacing of the twopressing surfaces from one another. The cold weld along the entire widthof the plastic film therefore has the same thickness as the plastic filmitself. The weld therefore has no raised shape and is subject to onlylow wear even given a high usage of the endless band as intermediatecarrier band in a printer or copier. As has been shown in practice, theweld is so uniform that this region can be fully utilized as afunctional surface given the function as an intermediate carrier band.As, for example, a photoconductive intermediate carrier band, the regionof the weld can be coated with a photoconductive layer within whichlatent image structures form due to exposure. This region of the weldcan likewise be utilized given employment of the endless band as anintermediate carrier band for the transfer of toner images, for examplefor superimposed toner images as well. It thus follows that an endlessintermediate carrier band manufactured in this way can have its surfacefully utilized, as a result whereof its overall length can be short andfurther design advantages derive in the structuring of the printer orcopier.

[0016] The plastic film has at least the width of a standard printingformat, i.e. at least the width of a DIN A4 sheet. The pressing surfacesshould also be correspondingly designed in terms of their respectivelength. As a result of the guidance by the pressing surfaces, it ispossible to produce a uniform and functional weld along this relativelygreat width. The force exerted on the pressing surfaces is to beempirically determined. It is dependent on the type of plastic film, onthe thickness of the plastic film and on the length of the weld to beproduced.

[0017] According to a further aspect of the invention, a device isrecited for manufacturing an endless band of thermoplastic plastic foran intermediate carrier band in an electrographic printer or copier. Theadvantages obtainable with this device agree with the advantagesdescribed for the method.

BRIEF DESCRIPTION OF THE INVENTION

[0018] The invention is explained in greater detail below on the basisof exemplary embodiments according to the Figures of the drawings.

[0019]FIG. 1 is a schematic illustration of a first exemplary embodimentof an inventive device;

[0020]FIG. 2 is a schematic illustration of a second exemplaryembodiment of an inventive device;

[0021]FIG. 3 is a schematic illustration of a part of an inventivedevice that can be additionally employed given the exemplary embodimentsaccording to FIGS. 1 and 2;

[0022]FIG. 4 is a schematic illustration of an inventive deviceaccording to FIG. 1 with two absorption devices, as fourth exemplaryembodiment;

[0023]FIG. 5 shows an absorption device composed of CrNi steel sheethaving an absorption layer; and

[0024]FIG. 6 shows an absorption device with a transparent glass pane, aDLC layer and an anti-adhesion coating.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Given the exemplary embodiment according to FIG. 1, athermoplastic plastic film 10 is placed between a transparent mountingelement, for example a glass pane 11, and a transparent counter-mountingelement, for example also a glass pane 12, being placed such that thefilm ends have their end faces lying exactly blunt against one another.Pressing frames 13 and 14 are provided for fixing the plastic film 10 aswell as for securing a smooth, non-raised weld, these exerting aprescribed force F onto the glass panes 11 and 12 and, thus, on to theplastic film 10. The flat surfaces of the glass panes 11 and 12 lyingagainst the plastic film 10 form planar pressing surfaces 11 a and 12 a.Alternatively, these pressing surfaces 11 a and 12 a can also beconcentric, for example cylindrical surfaces. The corresponding glasspanes are then elements of generated cylindrical surfaces. In adirection perpendicular to the paper plane, the plastic film has a widthof at least the width of a standard printing format, for example DIN A4.The glass panes 11 and 12 have a length that is greater than this width.

[0026] Via radiation-conducting fibers 15 and 17 as well as focussingoptics 16 and 18, radiation is supplied from radiation sources (notseparately shown) for heating the plastic film 10 beyond the meltingpoint in the region of the adjoining film ends. A weld 19 arises betweenthe film ends as a result thereof. The radiation sources are preferablylaser radiation sources, for example instance diode lasers, solid-statelasers, gas lasers or laser diode arrays. Dependent on the absorptivityof the material of the plastic film 10, a specific part of the radiationis absorbed and converted into heat. The pressing surfaces 11 a and 12 ahave a spacing from one another that is determined by the thickness ofthe plastic film 10 in its cold condition. This spacing is preservedwhen the ends of the plastic film 10 are heated and they melt, i.e. theforce F is selected correspondingly high. The molten material thendistributes along the bluntly abutting ends with a thicknesscorresponding to this spacing.

[0027] As a result of simultaneous irradiation of the plastic film 10from both sides via the focussing optics 16 and 18, a uniform weld 19can be achieved over the entire thickness of the plastic film 10, whichis especially advantageous given film materials with good absorbency.The counter-mounting element 12 is then composed of a material that istransparent for the radiation, for instance glass. This is particularlyadvantageous given film materials with a small penetration depth of theradiation that is less than half the film thickness. A noteworthytransmission part of the radiation is then no longer present.

[0028] For improving the quality of the welding process and forcompensating film material fluctuations, it is also expedient to measurethe temperature in the region of the weld 19. In a control circuit, thetemperature can then be kept constant at a defined value by modifyingthe radiation capacity.

[0029] In the exemplary embodiment according to FIG. 2, whereinidentical elements are provided with the same reference characters, asin the other Figures, a radiation source is provided at only one side ofthe plastic film 10 as well as the mounting element 11 and the pressingframe 13, the radiation source supplying radiation for the weld 19 viathe radiation conducting fiber 15 and the focussing optics 16.

[0030] Given a material of the plastic film 10 that is largelyimpermeable for the radiation and a transparent counter-mounting element12, a check is additionally implemented in this exemplary embodiment asto whether a gap is still present between the ends of the plastic film10 to be welded. To this end, a radiation detector 20, for example aphotodiode, is arranged at that side of the plastic film 10 facing awayfrom the irradiated side, the radiation detector 20 acquiring aradiation part that potentially passes through an existing gap. Theradiation part that passes through is nearly zero only given an exactpositioning of the ends of the plastic film. The exact positioning ofthe ends of the plastic film 10 can be implemented manually orautomated, whereby the radiation part that passes through should beminimal.

[0031] In a further exemplary embodiment, the thickness of the plasticfilm 10 and the radiation delivered by the radiation sources 15 and 16are matched such to one another that the optical penetration depth ofthe radiation is less than or equal to half the thickness of the plasticfilm 10. It is thereby assured that sufficient energy can be supplied tothe plastic film 10 in order to be able to correctly weld it.

[0032] For improving the efficiency, one of the mounting elements 11 and12, preferably the counter-mounting element 12, can be fashioned to bereflective at the appertaining pressing surface 12 a. Transmittedradiation is then reflected back into the plastic film 10. The mountingelement 12 can, for example, be fashioned as a mirror or as a polishedmetal sheet, preferably a copper or aluminum sheet, or can comprise areflection-coated material.

[0033] In the above-described exemplary embodiments according to FIGS. 1and 2 as well as in the exemplary embodiment according to FIG. 3 that isyet to be described, the mounting element 11 and the counter-mountingelement 12 can be provided with an anti-adhesion coating (not separatelyshown), for example Teflon or a hydrophobic DLC coating, at the side ofthe plastic film 10. A sticking of the plastic film during the weldingprocess is thus avoided.

[0034] In order to assure a qualitatively high-grade weld given thedevice according to FIG. 2 with an irradiation from only one side, theplastic film 10 can also be turned over, so that an irradiation fromboth sides ensues successively.

[0035] In the exemplary embodiment according to FIG. 3, whereinidentical elements are provided with the same reference characters as inFIGS. 1 and 2, an additional clamping of the plastic film 10 is providedwith the assistance of a rigidly seated clamp element 30 and a movableseated clamp element 31. The motion of these clamp element 30 and 31 forpressing the end of the plastic film 10 together is indicated with anarrow A. In other exemplary embodiments, both the clamp element 30 aswell as the clamp element 31 can be movable seated. The quality,particularly the strength of the weld, can be improved by pressingtogether with the assistance of the clamp elements 30, 31.

[0036] The inventive device with the plastic film 10 fixed in it can bemoved past under the focussed radiation in a direction perpendicular tothe plane of the drawing with a linear table. It is likewise possible tomove the radiation across the film region that is to be welded and thatis fixed in the inventive device. However, the radiation source, i.e.the radiation-conducting fiber 15 or, respectively, 17 as well as thefocussing optics 16 or, respectively, 18 must then be fashioned movable.Further, the radiation can be deflected, for example with a galvanometermirror, or can be expanded, for example with a linear optics. In thelatter case, a simultaneous welding over the entire film width ispossible.

[0037] In the exemplary embodiment according to FIG. 4, whereinidentical elements are provided with the same reference characters as inFIG. 1, a respective absorption device 40 and 42 is additionallyintroduced between plastic film 10 and mounting element 11 and 12. Theabsorption device 40 is located directly between the mounting element 11and the plastic film 10 and forms the first pressing surface 11 a. Theabsorption device 42 lies directly between the counter-mounting element12 and the plastic film 10 and forms the second pressing surface 12 a.That side of the absorption device 40 and 42 facing toward the radiationsource 16 or, respectively, 18 respectively absorbs the emitted radiantenergy and converts it into heat that is transmitted onto the ends ofthe plastic film 10 residing opposite one another and effects thewelding. In this exemplary embodiment, thus, arbitrary thermoplasticmaterial can be employed regardless of the respective absorptivity, forexample completely transparent plastic film.

[0038] In a further exemplary embodiment according to FIG. 5, theabsorption device 40 and 42 is composed of a thin metal sheet, forexample CrNi sheet steel, that is arranged between the plastic film 10and the mounting element 11 and 12. That side of the metal sheet 52facing toward the irradiated side can be roughened for improvedabsorption or can be provided with an absorbent coating 50, particularlywith black chromium or stove enamel.

[0039] In the exemplary embodiment according to FIG. 6, the absorptiondevice 40 and 42 is composed of an absorbent layer 62, particularly ahydrophobic DLC layer or a hard-aggregate layer, preferably respectivelyapproximately 0.2-3 μm thick, on a transparent glass pane 60 serving ascarrier. The glass pane 60 simultaneously assumes the function of themounting element 11 or 12 (see FIG. 4). The absorbent layer 62 can beadditionally provided with an anti-adhesion layer 64, particularly a DLCcoating, Teflon or silicone, preferably having a thickness ofapproximately 0.5-3 μm, at its side facing away from the radiation. Asticking of the plastic film 10 during the welding process is thusavoided.

[0040] The inventive method and the inventive device can be generallyapplied for all thermoplastics. The employment of polyester,polycarbonate or polyamide is especially beneficial, potentially withabsorbent additives for balancing the penetration depth of the radiationto be absorbed. Lampblack-filled polyamide or polycarbonate have therebyproven beneficial. The film thickness lies in the range from 50 to 200μm.

[0041] The inventive method and the inventive device serve for themanufacture of endless photoconductor bands, what are referred to as OPCbands (organic photoconducting material), as well as transfer bands forelectrophotographic devices. The weld is very uniform and has the samethickness as the plastic film. As a result thereof, it is also possibleto employ the region of the weld as a latent image carrier or as a tonerimage carrier. An endless band manufactured in this way can thereforehave a short length and the wear in the region of the weld is reduced.

1. Method for manufacturing an endless band of plastic for anintermediate carrier band in an electrographic printer or copier,whereby the ends of a thermoplastic plastic film (10) that comprises atleast the width of a standard printing format have their end facesplaced abutting one another, the ends of the plastic film (10) areheated by radiation to a temperature required for the welding, andwhereby a respective pressing surface (11 a, 12 a) is arranged at bothsides of the ends, the length of said pressing surface at leastcorresponding to the width of the standard printing format and thispressing the surfaces of the ends against one another such that, whenthe plastic material of the end faces of the ends residing opposite oneanother melts, the spacing of the pressing surfaces (11 a, 12 a) definedby the thickness of the cold plastic film (10) is preserved.
 2. Methodaccording to claim 1, characterized in that the plastic film (10) isheated by radiation proceeding from one side.
 3. Method according toclaim 1, characterized in that the plastic film (10) is heated byradiation from both sides.
 4. Method according to claim 3, characterizedin that the plastic film (10) is simultaneously heated by radiation fromboth sides.
 5. Method according to claim 3, characterized in that theplastic film (10) is heated by radiation at one side and, after beingturned over, is subsequently heated further at the other side.
 6. Methodaccording to one of the claims 1 through 5, characterized in that theheating of the plastic film (10) ensues by means of laser radiation. 7.Method according to one of the claims 1 through 6, characterized in thatthe radiation is respectively absorbed at that side of the plastic film(10) facing toward the radiation source (15, 17).
 8. Method according toone of the claims 1 through 7, characterized in that the ends of theplastic film (10) residing opposite one another are pre-stressedrelative to one another during the welding.
 9. Method according to oneof the claims 1 through 8, characterized in that the temperature of theplastic film (10) at the weld (19) is measured during the weldingprocess, and the radiation capacity is regulated dependent on themeasured temperature for keeping the temperature constant.
 10. Methodaccording to one of the preceding claims 1 through 9, characterized inthat the plastic film (10) is irradiated proceeding from one side andthe radiation passing through a gap between the ends to be welded ismeasured at the other side.
 11. Method according to one of the precedingclaims, characterized in that the pressing surfaces (11 a, 12 a) areformed by plates.
 12. Method according to claim 11, characterized inthat at least one of the plates (11, 12) is composed of a materialtransparent for the radiation, preferably glass, whereby preferablycomprise an anti-adhesion layer of Teflon or a hydrophobic DLC layer.13. Method according to one of the preceding claims, characterized inthat the thickness of the plastic film (10) and the radiation deliveredby the radiation source (15, 16) are matched to one another such thatthe optical penetration depth of the radiation is less than or equal tohalf the thickness of the plastic film (10).
 14. Method according to oneof the preceding claims, characterized in that an absorption device (40,42) for absorbing rays is provided on at least one side of the plasticfilm (10) and lying thereagainst.
 15. Method according to claim 14,characterized in that the absorption device (40, 42) lies in directcontact against the plastic film (10).
 16. Method according to claim 14or 15, characterized in that the absorption device (40, 42) forms thepressing surface (11 a, 12 a).
 17. Method according to one of thepreceding claims, characterized in that the absorption device (40, 42)is fashioned as metal sheet (52), preferably as CrNi steel sheet. 18.Method according to claim 17, characterized in that the metal sheet (52)preferably carries an absorbent coating (50), preferably of blackchromium or stoving enamel.
 19. Method according to one of the precedingclaims 17 or 18, characterized in that the side of the metal sheet (52)facing toward the radiation source is roughened.
 20. Method according toone of the preceding claims, characterized in that the absorption device(40, 42) is provided with an absorbent hard-aggregate layer or absorbentDLC layer, preferably having a thickness of 0.2-3 μm.
 21. Methodaccording to one of the preceding claims, characterized in thatpolyester, polycarbonate or polyamide is employed as plastic film. 22.Method according to one of the preceding claims, characterized in thatthe plastic film is composed of polyamide filled with lampblackparticles.
 23. Method according to one of the preceding claims,characterized in that the width of the standard printing format for theplastic film (10) at least corresponds to the DIN A4 format.
 24. Methodaccording to one of the preceding claims, characterized in that theplastic film (10) has a thickness in the range from 20 through 500 μm,preferably in the range from 50 through 200 μm.
 25. Device formanufacturing an endless band of plastic for an intermediate carrierband in an electrographic printer or copier, whereby the ends of athermoplastic plastic film (10) that comprises at least the width of astandard printing format have their end faces placed abutting oneanother, means are provided that heat the ends of the plastic film (10)by radiation to a temperature required for the welding, and whereby arespective pressing surface (11 a, 12 a) is arranged at both sides ofthe ends, the length of said pressing surface at least corresponding tothe width of the standard printing format and this pressing the surfacesof the ends against one another such that, when the plastic material ofthe end faces of the ends residing opposite one another melts, thespacing of the pressing surfaces (11 a, 12 a) defined by the thicknessof the cold plastic film (10) is preserved.
 26. Device according toclaim 25, characterized in that the plastic film (10) is heated byradiation proceeding from one side.
 27. Device according to claim 25,characterized in that the plastic film (10) is heated by radiation fromboth sides.
 28. Device according to claim 27, characterized in that theplastic film (10) is simultaneously heated by radiation from both sides.29. Device according to claim 27, characterized in that the plastic film(10) is heated by radiation at one side and, after being turned over, issubsequently heated further at the other side.
 30. Device according toone of the claims 25 through 29, characterized in that the heating ofthe plastic film (10) ensues by means of laser radiation.
 31. Deviceaccording to one of the preceding claims, characterized in that theradiation is respectively absorbed at that side of the plastic film (10)facing toward the radiation source (15, 17).
 32. Device according to oneof the preceding claims, characterized in that the ends of the plasticfilm (10) residing opposite one another are pre-stressed relative to oneanother during the welding.
 33. Device according to one of the precedingclaims, characterized in that the temperature of the plastic film (10)at the weld (19) is measured during the welding process, and theradiation capacity is regulated dependent on the measured temperaturefor keeping the temperature constant.
 34. Device according to one of thepreceding claims, characterized in that the plastic film (10) isirradiated proceeding from one side and the radiation passing through agap between the ends to be welded is measured at the other side. 35.Device according to one of the preceding claims, characterized in thatthe pressing surfaces (11 a, 12 a) are formed by plates.
 36. Deviceaccording to claim 35, characterized in that at least one of the plates(11, 12) is composed of a material transparent for the radiation,preferably glass, whereby preferably comprise an anti-adhesion layer ofTeflon or a hydrophobic DLC layer.
 37. Device according to one of thepreceding claims, characterized in that the thickness of the plasticfilm (10) and the radiation delivered by the radiation source (15, 16)are matched to one another such that the optical penetration depth ofthe radiation is less than or equal to half the thickness of the plasticfilm (10).
 38. Device according to one of the preceding claims,characterized in that an absorption device (40, 42) for absorbing raysis provided on at least one side of the plastic film (10) and lyingthereagainst.
 39. Device according to claim 38, characterized in thatthe absorption device (40, 42) lies in direct contact against theplastic film (10).
 40. Device according to claim 38 or 39, characterizedin that the absorption device (40, 42) forms the pressing surface (11 a,12 a).
 41. Device according to one of the preceding claims,characterized in that the absorption device (40, 42) is fashioned asmetal sheet (52), preferably as CrNi steel sheet.
 42. Device accordingto claim 41, characterized in that the metal sheet (52) preferablycarries an absorbent coating (50), preferably of black chromium orstoving enamel.
 43. Device according to one of the preceding claims 41or 42, characterized in that the side of the metal sheet (52) facingtoward the radiation source is roughened.
 44. Device according to one ofthe preceding claims, characterized in that the absorption device (40,42) is provided with an absorbent hard-aggregate layer or absorbent DLClayer, preferably having a thickness of 0.2-3 μm, whereby ananti-adhesion layer having the thickness 0.5-3 μm is preferably appliedonto the absorption device, whereby an anti-adhesion layer having thethickness 0.5-3 μm is preferably applied onto the absorption device.[sic]
 45. Device according to one of the preceding claims, characterizedin that polyester, polycarbonate or polyamide is employed as plasticfilm.
 46. Device according to one of the preceding claims, characterizedin that the plastic film is composed of polyamide filled with lampblackparticles or polycarbonate.
 47. Device according to one of the precedingclaims, characterized in that the width of the standard printing formatfor the plastic film (10) at least corresponds to the DIN A4 format. 48.Device according to one of the preceding claims, characterized in thatthe plastic film (10) has a thickness in the range from 20 through 500μm, preferably in the range from 50 through 200 μm.